As I said, if he develops into a Lindholm like player with less offensive upside, that's still a top 3 defenseman. Now, I don't see him reaching that level personally, and I don't necessarily disagree with everything you're saying. However, I think you're taking subtle jabs at Pettersson in your conversations about the trade. Maybe they're not intentional, but "he's a safer player" and "he's a depth defenseman" are not remotely similar statements. Like I said, it's not hard to see why Pens fans were taking some of your statements as jabs at them.
Your examples and reasoning seem a bit flawed to me. "tore up a junior league, lead a PP, or played top line minutes" are usually not going to be things someone with Pettersson's skill set shows in lower levels. I don't recall Hampus ever doing any of these things either. Though, he did have an impressive WJC. By your definition every offensive defenseman has more potential than more defense minded defenseman who don't put up great stats at lower levels. I completely disagree with that logic. Dumba, Pouliot, Trouba, Ceci, etc. all arguably had better metrics using your examples. Due to the age these guys are drafted, some are more project type players. That's clearly what Pettersson is; though he's already NHL level.
I do think Sprong was the better prospect, but I don't see near as much wrong with this trade from a Pittsburg perspective as you're implying. Definitely don't see this as Pittsburgh trading a high end prospect for a "serviceable depth d man". Just kinda weird that you're referring to Sprong not only as a prospect, but a high end one; yet Marcus is already just a serviceable depth d man...